PD drop over tails to CU

[davelittlewood]

House build in mid 80's
Single Phase 240V installation with main supply board in exterior wall enclosure.
CU mounted in downstairs toilet in the middle of the house. Main tails length in excess of 7m.

1) During the installation of the house alarm the alarm fitter (Qualified Electrician) commented that the main tails to the CU were small.
2) If the cooker and extractor fan are running and then the kettle is turned on then the fan speed noticeably drops.

I have measured the drop in pd at the CU (21:30 tonight).
With almost no load = 242V
With ~4kW load = 237V.

(I guess that grid pd is up now as it's catching up with Sunday lunch load!)

Is a drop in 5V across the tails something that needs attention?

I'm well aware of Part P so I'd get a Qualified Electrician to actually replace the tails if needed.
Just looking for some advice.

With a drop of 5V across the tails, the power consumed is about 85W.

Any help appreciated.

Cheers,

Dave

 

[Marvo]

Why do you suspect the 5V is being dropped across the tails?

 

[davelittlewood]

I measured the pd at the main switch in the CU with and without load.
I guess it could be in the single phase supply to the house.
I'll do the same test at the main supply board at the side of the house

 

[Marvo]

You're not measuring the volt drop across the tails by doing the tests you describe. The test would kinda moot, the wiring manufacturers publish volt drop figures for the wiring. If the tails are undersized just get them replaced.

 

[Octopus]

Hum...tails too small? What actual size are they?

 

[davelittlewood]

The tails are actually T+E, can't get it in to see the writing on the sheath or measure directly but the overall width of the outer is about 20mm so from various tables I think that it's 16mm^2.

The house originally had 2 x 8kw showers, 7kW cooker + usual lighting and ring main circuits.

 

[davesparks]

Are you sure that T&E is the tails? Is it not fed from - fuse or mcb in the main supply board you described? If it is fed from another board then that changes the situation

 

[davelittlewood]

As far as I can tell it's T+E from the mcb on the main supply board all the way to the CU.
I can see the T+E leaving the mcb and coming into the CU.

It comes in from the top, curls round behind the main switch and then pops back up again.

 

[davesparks]

In which case, that isn't the tails. That is what is called a sub-main or distribution circuit.
Tails are the cables which connect between the meter and the main switch or first fuseboard.

A little bit of voltage drop when loads are applied is not uncommon in the suppliers distribution cables, especially on long cable runs.

Where I live it is quite normal to see the lights dim when the kettle is switched on as we are in a rural location on an overhead line from a small pole mount transformer.

 

[Dave OCD]

That looks like 16mm to me, you mention an MCB at the main supply end, what is the current rating ? I'd be more used to seeing a switch fuse set up than a MCB at the source for a sub-main.

 

[davelittlewood]

Sorry, yeah you're right, 80A fuse and switch.

I also want to run a 3kW single phase compressor in the garage (which is currently an unfused spur on the kitchen ring main) so I'm concerned that the 16 sq mm from the switch to the CU isn't up to it.

As I said, I'm not an electrician, hence the request for some advice before I start getting quotes to put in larger cable.

As the CU is right in the middle of the house it's going to be a PITA to get access to run the new cable.

 

[davesparks]

Well this thread is taking a rapid change of direction isn't it.

We've gone from a concern over a bit of voltage drop to advice on how to proceed with a supply for a compressor!

I would be more concerned about the unfused spur than the 16mm submain.

You need an electrician to visit and make an assessment of what is needed and provide a quote.

 

[Octopus]

What do you mean by the "main supply board"?

Can you post a picky?

 

[Dave OCD]

A 7 metre run of 16mm T&E ought to cope fairly comfortably with the demand in an average home, the volt drop under fairly small loads does seem a little high though, that could be indicative of a poor connection anywhere upstream of your CU or a number of other things,might be nothing at all wrong even.
The 3kW compressor should be fine IMO, but ideally it would be better if the garage was on its own circuit, would that also be an awkward cable run from the CU ?
All things considered your best bet would be to get a decent electrician around to check it out properly and discuss the best course of action.

 

[davelittlewood]

We're going to rebuild the garage (as the foundations are knackered) so I'm already planning to get a sparky to put in a dedicated circuit for the garage rather than the spur from the kitchen ring main.

Picky below of the board with the main switch, meter etc.
The mcb in the bottom left supplies a separate circuit for my car lift (this might end up being the circuit for the complete garage as it's over spec'd for the load. I asked the electrician to put in a larger cable just in case).

You can see the T+E leaving top left.

 

[Octopus]

W

Picky below of the board with the main switch, meter etc.
The mcb in the bottom left supplies a separate circuit for my car lift (this might end up being the circuit for the complete garage as it's over spec'd for the load. I asked the electrician to put in a larger cable just in case).

You can see the T+E leaving top left.

View attachment 30381

Thats a bit of a dogs dinner isn't it!

 

[Dave OCD]

With all due respect the supply job for your car lift looks a lash up , the supply to the enclosure looks like 6mm straight off the outgoing side of the house sub main switchfuse - and apart from the MCB in the enclosure bottom left of the photo there should be a proper DP isolator, depending on the cable run/when the work was done there should really be an RCD also.

 

[davesparks]

With all due respect the supply job for your car lift looks a lash up , the supply to the enclosure looks like 6mm straight off the outgoing side of the house sub main switchfuse - and apart from the MCB in the enclosure bottom left of the photo there should be a proper DP isolator, depending on the cable run/when the work was done there should really be an RCD also.

The mcb is after the main switch so no other isolator is needed.
And SWA rarely requires RCD protection

 

[Dave OCD]

But is it really good practice to have to isolate the whole house in order to properly isolate the other separate supply , the thoughts on RCD protection were based on what's at the other end but of course I haven't seen the other end of the SWA.

 

[davesparks]

But is it really good practice to have to isolate the whole house in order to properly isolate the other separate supply , the thoughts on RCD protection were based on what's at the other end but of course I haven't seen the other end of the SWA.

It's no different to feeding it from a way in a CU.
DP isolation is only required for the main switch, after that single pole isolation is all that is required.

There is a car lift on the other end of it.